In fiber-optics, a polarization-maintaining optical fiber (PMF or PM fiber) is a single-mode optical fiber in which linearly polarized light if properly launched into the fiber maintains a linear polarization during its propagation. The polarized light thus exits the fiber in a specific linear polarization state so there is essentially no cross-coupling of optical power between the two orthogonal polarization modes. Such fiber is used in special applications where preserving polarization is essential. PM fiber amplifiers use PM fibers and are needed in some high reliability applications in which the fiber amplifier is required to operate with as well as maintain a specific state of polarization of the received optical launch signal.
State-of-the-art PM amplifiers involve an all-PM fiber structure incorporating fiber components built with PM fibers as well as a PM doped fiber. The PM fiber includes but is not limited to PANDA fibers that have a step index profile and elliptical core fibers. Variants of these PM fibers include PM fibers that allow transmission in a specific polarization axis or non-polarizing which allow transmission on both polarization axes.
There are several known methods for pumping PM fiber amplifiers that use first and second pump lasers emitting at the same wavelength (e.g., electrically pumped laser diodes providing an emission at 980 nm for an Erbium Doped Fiber Amplifier (EDFA) which emits amplified light within the 1525-1565 nm band. Co-propagating (or forward) pumping is known for a low Noise Figure (NF), but results in low power conversion efficiency (PCE) and a low gain. Forward pumping means that the pump wave travels in the same direction as the signal wave. Counter-propagating (or backward) pumping is known for a high NF with high PCE and high gain. Backward pumping means that the pump wave travels in the opposite direction as the signal wave. Bi-directional pumping involves pumping in both the forward and backward directions simultaneously to realize the advantages of both the co-propagating and counter-propagating schemes.
Bi-directional pumping is thus often desirable since it can maintain a low NF and at the same time achieve a high PCE. However, optical cross-talk between co-propagating and counter propagating pump lasers can occur. Optical isolators such as Faraday rotators which allow light to pass only in a single direction are known to provide protection of active devices such as lasers against injection of unwanted light. However, optical isolators are generally expensive, lossy and bulky components, in particular when used for isolating light in the short wavelengths. Therefore the deployment of optical isolators is generally not practical from the system point of view in terms of cost, efficiency and size.
One other solution to mitigate the problem of pump laser crosstalk in bi-directionally pumped PM fiber amplifiers involves using pump lasers emitting at different wavelengths. These pump lasers typically employ fiber Bragg grating filters for stabilization to filter-out/reject incident signals of a wavelength outside the Bragg grating filter's pass-band.